Various methods are known for removing liquid from mixtures of solids and liquid. In cases where solids consist primarily of compressible granular, fibrous or cellular material and the liquid is water, equipment selected for these applications have included belt presses, centrifuges, screw presses and vacuum filters.
There are technical and economic reasons for selecting a particular technology. Technical reasons for selecting a belt press include the ability to operate continuously; the ability to produce a dry filter cake with a target specification for % solids (or % moisture equal to 100%-% solids); and the ability to retain a high proportion of the incoming solids in the filter cake.
Numerous examples of belt presses are provided in the prior art—for example, U.S. Pat. Nos. 6,454,102 (Thompson), 5,592,874 (Blauhut), 5,545,333 (Louden et al.) and 4,681,033 (Crandall et al.). In general, a belt press has a frame with an upper belt and a lower belt. Both upper and lower belts are typically liquid permeable [see U.S. Pat. Nos. 6,454,102 (Thompson), 5,592,874 (Blauhut), 5,545,333 (Louden et al.), 5,109,764 (Kappel et al.), 4,986,910 (Uyama et al.), 4,961,862 (Janecek), 4,879,034 (Bastgen), 4,836,100 (Johnson et al.), 4,681,033 (Crandall et al.), 4,181,616 (Bahr), 4,147,101 (Heissenberger et al.), and 3,942,433 (Wohlfarter)], although U.S. Pat. No. 5,022,989 (Put) suggests a non-permeable upper belt combined with a permeable lower belt is more advantageous. U.S. Pat. No. 4,986,910 (Uyama et al.) discusses the limitation of twin permeable filter fabrics for dewatering sludges.
Belt presses can employ several stages at which different techniques are used to remove the liquid from a mixture of solids and liquid. The use of these different stages improves filtering efficiency throughout the belt filter press.
In a typical belt press with multiple stages, the mixture initially enters a gravity screening zone of the belt press where free water drains through the lower permeable belt by gravity. The gravity screening zone is used primarily to remove free water. At the end of the gravity screening zone, the mixture could be near 10% solids. U.S. Pat. Nos. 6,454,102 (Thompson), 5,545,333 (Louden et. al.), 5,240,609 (Langley), 5,066,399 (Sugihara et al.), 5,022,989 (Put), 4,986,910 (Uyama et al.), 4,961,862 (Janecek), 4,836,100 (Johnson et al.) and 4,181,616 (Bahr) describe belt presses with gravity screening zones.
After the gravity screening zone, the mixture of solids and liquids enters the wedge zone where the upper belt and the lower belt carrying the mixture of solids and liquid converge with one another, applying progressively increasing pressure as the mixture is compressed between the belts. The degree to which the mixture is compressed corresponds to the angle of convergence of the belts, also called the wedge angle. The wedge angle may be fixed at all times or mechanically adjusted prior to running the filter press to accommodate a particular mixture. Wedge zones are shown in U.S. Pat. Nos. 6,454,102 (Thompson), 5,545,333 (Louden et al.), 5,240,609 (Langley), 5,109,764 (Kappel et. al), 5,066,399 (Sugihara et al.), 5,022,989 (Put), 4,986,910 (Uyama et al.), 4,961,862 (Janecek), 4,879,034 (Bastgen), 4,836,100 (Johnson et al.), 4,681,033 (Crandall et al.), 4,181,616 (Bahr), 4,053,419 (Pav), 3,942,433 (Wohlfarter), 3,894,486 (Sparowitz et al.) and 3,796,149 (Heissenberger).
Some belt presses have a higher pressure zone wherein the upper and a lower belt follow a path between progressively smaller press rolls which may be either perforated or solid drums. The pressure imparted to the material between the upper and lower belts in the high pressure stage increases greatly from the largest perforated drum or solid roller to the smallest perforated drum or roller. High pressure zones are shown in U.S. Pat. Nos. 5,545,333 (Louden), 5,240,609 (Langley), 5,066,399 (Sugihara et. al), 4,986,910 (Uyama et. al), 4,879,034 (Bastgen), 4,836,100 (Johnson et al.), 4,681,033 (Crandall et al.), 4,181,616 (Bahr), 3,942,433 (Wohlfarter), and 3,796,149 (Heissenberger).
Some practitioners prefer the use of pressure plates instead of press rolls in the wedge zone and/or high pressure zone as described in U.S. Pat. Nos. 5,592,874 (Blauhut), 5,109,764 (Kappel et al.), 4,879,034 (Bastgen), 3,998,149 (Malarkey) and 3,945,789 (Boman).
U.S. Pat. No. 5,545,333 (Louden et. al) reported dried cake from a belt press is typically 30% solids.
U.S. Pat. No. 5,066,399 (Sugihara et. al) reported dried cake using a belt press designed with prior art was 70-75% moisture (25-30% solids) for an industrial mixture with initial feed of 90-95% moisture (5-10% solids); while a belt press incorporating the '399 patent produced a cake with 65-70% moisture (30-35% solids). It was also reported in the '399 patent that a belt press designed with prior art produced a dry cake with 80-90% moisture (10-20% solids) for a photo-etching mixture with initial feed moisture of 90-95% (5-10% solids), while the belt press designed with the '399 improvements produced a cake with 70-75% moisture (25-30% solids),
U.S. Pat. No. 4,986,910 (Uyama et. al) reported ranges for belt press performance from a medium compression wedge zone are 65-85% moisture (15-35% solids); and belt press performance from a high compression dewatering zone is 60-65% moisture (35-40% solids). The '910 patent compared the performance for dewatering sewage sludge with prior art as 68.6% moisture (31.4% solids); while the performance for dewatering the same sewage sludge using a belt press with the '910 patent improvement was 65.5% moisture (34.5% solids); and a belt press with the '910 patent improvement with coke dust added produced 58.8% moisture in the dried cake (42.2% solids).
There are two problems with all prior art. First, many applications require higher % solids in the dried cake than can be produced using prior art. Some landfills refuse to accept sewage sludges that do not pass a minimum % solids content or stickiness test, requiring municipalities to add drying sand beds or find more costly land spreading options for sludge disposal. Dewatered livestock manures require addition of bulking agents before composting because moisture content of dewatered sludge is too high. Ethanol processors incur higher operating expense when the moisture of wet distillers' grains from processing whole stillage is too high.
Second, the percentage of incoming solids to the belt press retained in the filter cake is too low. Manure processing and food waste processing require a very high percentage of solids retained in the filter cake in order to meet effluent discharge water quality criteria or avoid contamination of downstream processes. Typical results for belt presses were not reported in the prior art but are available in public documents like                (a) US EPA, Development Document for the Final Revisions to the National Pollutant Discharge Elimination System Regulation and the Effluent Guidelines for Concentrated Animal Feeding Operations, December 2002,        (b) U.S. EPA, Biosolids Technology Fact Sheet, “Belt Filter Press”, Document Number EPA 832-F-00-057, September 2000; and        (c) North Carolina—Development of Environmentally Superior Technologies (EST) for Swine Waste Management 2003-2006, Technology Report—Solids separation/nitrification-denitrification/soluble phosphorus removal/solids processing system, July 2004 update available at http://www.cals.ncsu.edu/waste_mgt/smithfield_projects/phase1report04/A.9Super%20S oil%20final.pdf.        
Credible, publicly available sources report a range of performance for percentage of incoming solids retained in the filter cake for belt presses as 40-85% which is either not acceptable or too costly for processing livestock manure, food waste, beer manufacturing, and other organic mixtures.
Therefore, the object of this invention is to provide a belt press design for thickening and dewatering to produce separated solids with solids content of 50-90% while retaining 99% of the incoming solids in the filter cake.